Germany's main center-left party faces pressure to decide on a challenger to Angela Merkel in next year's election after the chancellor announced she will seek a fourth term.

The Social Democrats lost the chancellery to the conservative Merkel in 2005 and currently serve as her junior coalition partners. They trail in polls, and their hopes appear to depend on a left-wing alliance with the Greens and the Left Party that would have to overcome significant policy divisions.

Party leader Sigmar Gabriel, who is also Merkel's vice chancellor and economy minister, has yet to decide whether to run. European Parliament President Martin Schulz is also considered a strong possible contender.

Germany's top labor union official, Reiner Hoffmann, called for "clarity" from the Social Democrats in comments Monday to the daily Tagesspiegel.